1. Field of the Invention
The present invention is directed to cellular telephone system call management. More particularly, the present invention is directed to a system and method for assigning about-to-be-handed off cellular calls to a neighbor cell site based on a criteria such as traffic load at the time of handoff.
2. Background of the Invention
FIG. 1 depicts a typical cellular telephone system in which a plurality of cellular antenna cell sites 100a, 100b, 100c are each capable of servicing a mobile telephone or other wireless devices such as a personal digital assistant (PDA) or interactive pager (hereinafter, collectively, “mobile device” 115). As mobile device 115 travels from one location to another it is necessary to hand off, from one cell site to another, the communication that is occurring between mobile device 115 and another party or entity. That is, as mobile 115 device travels, the signal to and from an “active” cell site, i.e., the cell site that is currently servicing the mobile device, will eventually fade, making it necessary to hand the communication off to another cell site whose signal strength at the mobile device's then-current location is greatest among a group of preselected cell sites. To accomplish this task a mobile switching center (MSC) 120 is provided to, among other things, assign the cell site to be handed off to based on feedback from mobile device 115. More specifically, MSC 120 determines which neighbor cell site has the greatest signal strength to the mobile device, thereby identifying the neighbor to hand off to.
FIG. 2 is a flow chart of a conventional mobile device handoff procedure. At step 201 mobile device 115 registers with a cellular network when, e.g., it is first powered up. Upon registration with the cellular network (i.e., the collection of cell sites 100a, 100b, 100c, MSC 120, etc.), MSC 120 generates a list of likely handoff cell site candidates, often called “neighbors.” The generated list is called the mobile assisted handoff (MAHO) list and includes a list of frequencies to which the mobile device should tune periodically to record the then-available signal strength. Typically, the MAHO list lists the digital control channel (DCCH) frequency of the likely hand off candidates, in accordance with the well-known IS-136 cellular communications specification. However, any time slot of a channel could be used to assess the signal strength from a particular cell site 100. At step 203, the generated MAHO list is passed back to the active cell site, which then passes the MAHO list to mobile device 115. At step 205, mobile device 115 monitors the signal strength of each of the frequencies in the MAHO list and then at step 207, mobile device 115 sends back to the active cell site and, ultimately, to MSC 120, at least a subset of the monitored signal strengths.
At step 209, MSC 120 evaluates the signal strengths that have been returned by mobile device 115 and orders or arranges them (logically) by signal strength, or at least identifies the cell site having the strongest signal strength among the group of MAHO-listed cell sites. Meanwhile, although not depicted in the flow chart, mobile device 115 is also continuously monitoring the signal strength of the cell site that is servicing mobile device 115. This information is also communicated back to MSC 120, as is well known in the art. Accordingly, at step 211, it is possible for MSC 120 to determine whether a neighboring cell site has a signal strength that is stronger than the servicing cell site. If the servicing cell site still has the strongest signal strength among those listed in the MAHO list, or the signal strength of the servicing cell site has not dropped below a predefined threshold, then mobile device 115 is deemed not to be ready for hand off and the process loops back to step 205. Under typical circumstances, MSC 120 is updated with signal strength information about once per second.
If, on the other hand, either the signal strength of one of the cell sites in the MAHO list is stronger than the active cell site or the signal strength of the active cell site has fallen below the predefined threshold, at step 213 MSC 120 determines if at the cell site identified in step 209 there is a channel available to receive a handoff. If, at step 215, a channel is available, handoff is executed at step 217 and a new MAHO list is provided to the mobile device at step 203.
If, at step 215, a channel is not available at the cell site identified in step 209 (i.e., all channels are being used on that cell site), MSC 120 checks the next entry in the ordered MAHO list, i.e., for the cell site having the next strongest signal strength, step 219. If, at step 221, there is no further cell site in the MAHO list or none of the those cell sites has a signal strength above the threshold, then at step 223 the call or radio link is dropped from the network altogether. If at step 221 there is another cell site to evaluate in the MAHO list, then the process returns to step 215 to evaluate that next cell site.
Thus, in accordance with the prior art methodology of handing off mobile device calls, the MSC is programmed to identify the neighbor cell site having the strongest available signal strength. However, this methodology can lead to an undesirable “pile-up” on selected cell sites, especially those cell sites that service areas of high congestion, e.g., during a rush hour period.